Ion implanters having different ion beam geometries are known in the prior art. Such ion implanters cause ions from an ion source to be accelerated to high-energies and impact one or more wafers positioned within an implantation chamber until a controlled implant dosage is achieved. Generally, wafers are implanted only about one third of the time the ion source is generating the ion beam. The remaining time is spent in moving wafers to and from the implantation chamber and performing other steps to prepare the implanter for ion implantation. This means that for considerable time periods the ion beam is collected in a Faraday cup which is placed in the ion beam path in close proximity to the implantation chamber. As the ion beam impacts the Faraday cup it causes the bombarded material to disintegrate. Ion sputtering at the region of the Faraday cup also contaminates other components of the ion implanter necessitating periodic beam shut-down and component replacement or cleaning.
With the move to ever finer design rules in semiconductor fabrication, the issue of contamination of the wafer by particles during implantation ("particulate" contamination) becomes an increasingly significant problem. For example, the present specification for an advanced implanter can require that less than 8 particulates of 0.3 micron size or larger be added to a 20 cm diameter wafer during the implant cycle. It is important to limit the number of particulates generated in the beam line, since these can be transported to the wafer region by the ion beam or by pumping and venting procedures. It is consequently important to keep the regions close to the process or implantation chamber as free of particles as possible.
The ion source is not turned off when ions are not treating wafers. It takes a considerable amount of time to readjust the source each time it is shut down so that instead, the ions are directed into the Faraday cup. It has been proposed to shut down the ion source during wafer handling and other non-implant intervals. Such proposals are not practical at the present stage of source development, however, and may not be the most efficient way to avoid the problems accompanying ion sputtering onto implanter surfaces.
In the typical ion implanter, a source emits ions which are accelerated and move along an initial trajectory leading to a beam-resolving magnet. The magnet bends the ion beam, causing ions emitted by the source to follow a specific trajectory. Downstream from the magnet the ions may be further accelerated to a desired implantation energy and directed into the implantation chamber. Ions emitted from the source having an inappropriate mass follow trajectories in the magnet that remove them from the primary ion beam. These ions strike implanter walls upstream from the ion implantation chamber. Ions having the appropriate mass either impact the wafers during implantation or are otherwise directed into the Faraday cup that is moved into the ion beam just upstream from the ion implantation chamber.